Available techniques within the present field involve a one step injection moulding process resulting in cover portions located inside as well as outside of the tearing denotation. The injection ingate generally is located centrally in the mould and the material flows more or less radially outwards in the cover to be moulded, across the tearing denotation and then continues essentially in the same direction for forming a flange to be attached to a container casing.
However, it has been recognized that the one step moulding technique does not provide an acceptable tearing possibility, especially when the plastics material is of the type forming long chains of oriented molecules when stretched or otherwise oriented. Unfortunately, the one step technique gives exactly such stretching/orientation in the tearing region of a cover of the actual type.
It is known that polypropylene, especially, is very difficult to tear. The tearing is made more difficult due to the fact that the molecule chains are oriented across the tearing denotation exactly in the regions where tearing is to be carried out. There is also suggested a sequential moulding process resulting in a weld zone of reduced strength defining a tearing denotation of a cover.
Within the present technical field, there have been proposed composite closures, for instance a cover having a relatively thick supporting and protecting part of plastics and a thin metal foil attached to said part. Such closures advantageously may be used in applications demanding extremely low gas and vapor transmission characteristics, as for instance is the case in packages of the retortable type.
There is also a need for providing easy opening facilities in such composite structures.
Available closures of this type are composite structures comprising a supporting and protecting plastics element, normally having a central panel and a circumferential flange, and a metal foil laminate attached to the plastics element. The panel is delimited from the flange by a denotation, usually comprising regions exposing the foil to the outside of the closure. Such exposed regions are very difficult to avoid when trying to arrange the structure for easy opening in the true meaning thereof.
Additionally, the need for exposed regions involves a puncture risk of the foil and lowers the strength of the overall container.
In practice, the need for exposed regions has prevented moulding of the protecting and supporting plastics part directly onto the thin metal foil. The foil is not able to withstand stretching and other strains that will occur when pressing down the foil by fingers or other mechanical means for forming exposed regions.
Notwithstanding the easy opening condition, there is also a pronounced need for an overall container structure that withstands internal pressure, for instance from carbonated soft drinks.